Litecoin, like any scrypt coin, hasn't been worth mining on GPUs for ages - much less CPUs which is the only option the QT client EVER supported.
I don't think CURRENT versions of the QT client support mining at all any more due to that.
ASIC took over all of the Scrypt coins a few years back.
Thanks for your reply.
But still I need to mine with multithread for experiment only. I am using Litecoin-qt v0.13.1 in which "generate" command is working but use only 1 physical core in my cpu. I want to use all physical cores. I know there are other third party cpuminer where we can use more than 1 core for mining but I want to specifically use Litecoin-Qt with multithreading.
Any workaround or command?